Developments in semiconductors in the last decade or so and recent changes in the allocation of portions of the radio frequency spectrum have contributed to the vigorous growth in the uses and markets for various portable transceivers, such as used in cellular telephone service and GMRS. That which is to be manufactured must also be tested and, as needed, repaired and tested. The advent of spread spectrum technologies, such as CDMA in the United States and GSM in Europe, require that additional levels of sophistication be included in the test equipment, to allow that aspect of the performance of the device under test to be evaluated. Such test equipment ought to be reliable and relatively compact (those wishing to sell systems comprising a rack full of separate instruments need not apply . . . ) as well as adaptable to future developments.
Among the things that such a test set has to do is measure RF power. The power measurement module of such a test set ought therefore to be small, accurate, broad band, suitable for pulse modulation applications in addition to CW, and, inexpensive. That is quite a list of disparate requirements, and poses a significant challenge to the designers of a test set to be used with radio equipment such as cellular telephones.
A calorimetric RF power meter is a device that accepts RF power into a terminating load, and thermally couples the heat generated to a temperature dependent resistance that is one arm of a bridge. This unbalances the bridge and produces an error signal within a servo loop. The servo responds by applying DC or low frequency power to a separate but identical terminating load whose heat is coupled to another arm of the temperature sensitive bridge. The applied power is measured by metering how much power is required of the servo loop to rebalance the bridge. Calorimetric RF power meters of this sort have been known for some time. See for example, the Operating and Service Manual for the Hewlett-Packard Model 434A Calorimetric Power Meter (circa 1961). A more recent example of this technique can be found in an article on page 26 of the July 1987 Hewlett-Packard Journal entitled "Microprocessor-Enhanced Performance in an Analog Power Meter".
The accuracy of calorimetric bridges is affected by temperature, and especially by temperature differentials occurring across the bridge. Even though the electrical value of the components within the bridge can be trimmed into balance, they still have temperature coefficients, and it is exceedingly difficult to control the thermal paths within the physical part. The result is that under a thermal gradient, such as the application of power to be measured, the bridge can become unbalanced owing to an unsymmetrical response of the bridge itself to the thermal gradient. In time, if steady state conditions are maintained, the gradient will level out and an accurate answer will be available. It would be desirable if this need for thermal time constant response time could be eliminated.
A control loop can use an amplifier or an integrator to form the feedback signal. In many situations where a small change is to be measured and a DC measurement technique is prone to unacceptable drift, it is advisable to shift to AC measurement techniques. In these cases a synchronous detector coupled to an integrator form an attractive combination for forming the actual feedback signal from the amplified error signal. If, as in the case of a calorimetric power measurement technique, the integrator output is used to produce heat, then either polarity of output from the integrator can cause a point of balance. However, one polarity has associated therewith the wrong logical sense of change as between the feedback signal and the error signal. If the system gets into that state the feedback is not longer negative, but becomes positive. As a result, it is generally necessary to anticipate a worst case of margins and then prevent the integrator output from getting within that close to zero, even when the bridge is actually balanced. As a complication to the logic of the servo loop, that brute force solution is indeed manageable, but it can nevertheless have the disadvantage of limiting the accuracy with which small signal levels can be measured. It would be desirable if this limit on dynamic range could be eliminated by allowing the integrator output to operate much closer to zero when the bridge is indeed balanced.